1. Field of the Invention
The present invention relates to the electrical connection of a high-voltage cable to a spark plug used in internal combustion engines of cars or other vehicles, stationary apparatus such as generators, or portable apparatus such as chain-saws, and the like. The invention more particularly concerns a shield or hood for covering the terminal portion of the high-voltage-cable.
2. Description of Background Information
FIGS. 1-7 show an example of the structure of a prior art plug connection device.
The device comprises a socket terminal 2 made of stainless steel or similar material and a shroud or boot 3 made of insulating elastic materials, such as silicone rubber or other elastomer, in which the socket terminal is contained. At one end of the socket terminal 2 a cable connecting portion 2a is provided where the high-voltage cable is crimped and electrically connected. The socket terminal 2 further has a plug-connecting portion 2b at the other end. This portion has an axis perpendicular to that of the cable connecting portion 2a and receives a bulb-like stud terminal 5a of a spark plug.
The insulating shroud 3 has a substantially L-shaped configuration, one branch of which forms a sleeve portion 3a into which the socket terminal 2 is received. The sleeve portion 3a defines a cable path-hole 3b through which the high-voltage cable is introduced. The other branch of the L-shaped shroud defines a tubular plug receiving portion 3d having a plug path-hole 3c and a closed end. The stud terminal 5a of the spark plug is guided through this path-hole 3c and inserted into the plug connecting portion 2b.
The shroud 3 is enclosed in a metal cover 7. As shown in FIG. 2, the metal cover 7 is prepared by stamping out an appropriate shape from a metal plate, i.e., an aluminum sheet, and folding or bending it. The central part of the metal plate corresponds to the top-cover portion 7a while the side parts flanking the top-cover portion 7a correspond to laterally-defining side cover portions 7b, 7c which are substantially symmetrical. The metal cover 7 is shaped by folding or bending these parts.
With a metal cover 7 as shown in FIGS. 1-5 and 7, the outer circular surface of the plug receiving portion 3d is covered with a pair of side-cover portions 7b, 7c which extend along the guiding direction of the stud terminal 5a. When the sidecover portions 7b, 7c are folded and shaped, they form joint seam zones 7d, 7e. One of them is provided with a recess 7f, while the other is provided with a complementary shaped tab 7g. Thus, the tab 7g is fitted into the recess, as shown in FIG. 5.
In this construction, the upper and lateral parts of the socket terminal 3a are shielded respectively with a top cover 7a and side-cover portions 7b, 7c. The side-cover portions are provided with corresponding fitting flanges 7h, 7i depending from the bottom end thereof. Normally the spark plug comprises a stud terminal 5a, a metallic stud cover 5b and a hexagonal handling nut 5c therebetween. When the stud terminal is inserted through the plug path-hole 3c and connected to the socket terminal 2, the hexagonal handling nut 5c is fitted between the flanges 7h, 7i.
As shown in FIG. 1, a dielectric sheet 10 is interposed between the external surface of the elastomeric shroud 3 and the internal surface of the metal shield 7.
When the spark plug 5 is mounted onto a side of an engine 9 and the stud terminal 5a is connected to the plug-connecting portion 2b of the plug connection, the flanges 7h, 7i of the metal cover 7 are put into contact with an outer surface of the hexagonal handling nut 5c. This cover 7 prevents thermal deteriorations of the elastomeric shroud 3. Such a type of flanged metal cover is described, for instance, in U.S. Pat. No. 5,348,486, issued on Sep. 20, 1994.
With the structure of the prior art cover, each flange 7h, 7i is put into contact with the hexagonal handling nut 5c and thus is electrically grounded to the side of the engine 9. The heat released is also led away. For this purpose, each flange 7h, 7i has to be in contact with an outer surface of the hexagonal handling nut 5c.
As mentioned above, the side-cover portions 7b, 7c have joint seam zones 7d, 7e correspondingly provided with a recess 7f and a tab 7g. The recess and the tab are interlocked by mutual engagement so that both joint seam zones 7d, 7e are firmly closed and resist a tangential opening force. Also, the diametric distance L separating both flanges 7h, 7i corresponds roughly to the diameter M of the tangent circle of the hexagonal handling nut 5c.
In this structure, the diametric distance L between the flanges 7h and 7i is determined solely on the basis of the diameter M of the tangent circle. Thus, when manufacturing, a corresponding dimensional precision is needed to fix that distance.
When the diametrical distance L is shorter than the diameter M of the tangent circle, the two flanges 7h and 7i may not fit properly onto the hexagonal handling nut 5c. Consequently, the elastomeric shroud 3 cannot be fitted smoothly over the spark plug 5, leading to a low work efficiency.
On the contrary, when the diametrical distance L is longer than the diameter M, the fit is loose and easily jolted. Consequently, engine 9 may shake the metal cover 7 off the hexagonal handling nut 5c, and further increase the misfitting. Sometimes, the metal cover 7 may separate from the elastomeric shroud 3.
Further, the metal cover 7 cannot exert the force necessary to fix the elastomeric shroud 3 onto the spark plug 5. Vibrations then increase the abrasion between the stud terminal 5a of the spark plug and the plug-connecting portion 2b, thus disrupting the electrical contact.